This invention relates to the reduction of color formation in hot melt adhesive formulations which contain diene polymers, especially block copolymers of conjugated dienes and vinyl aromatic hydrocarbons. More particularly, it relates to the reduction of such color formation by the addition of specific alkyltin compounds to such formulations.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprises primarily those having a general structure EQU A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as butadiene or isoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases--a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomena allows the A--B--A rubber to behave like a conventionally vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such in adhesive formulations; as moldings of shoe soles, etc.; impact modifier for polystyrene resins and engineering thermoplastics; modification of asphalt; etc.
Many such polymers are made by coupling with coupling agents as described in U.S. Pat. No. 4,096,203. Such coupling agents may contain halogens and often there is some residual halogen which reacts with residual lithium to form LiX.
Adhesive formulations containing coupled polymers containing LiI, LiCl or LiBr salts are known to develop a brown color when aged at high temperatures such as 177.degree. C. This problem does not occur in sequentially polymerized polymers. This is considered a problem for hot melt adhesive manufacturers who sometimes hold adhesives at high temperatures for extended times during hot melt application. The brown color is undesirable for many end uses for such adhesive products such as diaper assembly, clear labels, clear tapes, clear decals, etc.
I have found that color generation is not correlated with increased oxidative degradation. Thus, the commonly used methods for dealing with color formation in polymers are not effective in this situation. I have found that the use of specific alkyltin stabilizers will greatly reduce the color formation in such hot melt adhesive formulations.